1. Field of the Invention
The present invention relates to a semiconductor device and a photoelectric converting apparatus using the semiconductor device.
2. Related Background Art
As a conventional semiconductor device, an example of a bipolar transistor (hereinafter, referred to as a BPT) will be described. The BPT is mainly classified into a hetero bipolar transistor (hereinafter, referred to as an HBT) in which a semiconductor region of a wide gap is used for only an emitter and only the portion between the emitter and the base is constructed as a hetero junction and a double hetero BPT using a semiconductor region whose base has a narrower gap than that of the other emitter and collector. However, in both of them, the compositions in the horizontal direction of the base are constant.
FIG. 1 is a schematic cross sectional view showing an example of a conventional BPT. In the diagram, reference numeral 1 denotes a substrate (for instance, an Si semiconductor substrate); 2 indicates an n.sup.+ type buried region; 3 an n.sup.- region of a low impurity concentration; 4 a p region serving as a base region; 5 an n.sup.+ region serving as an emitter region; 6 a channel stop n.sup.+ region; 7 n.sup.+ region to reduce a collector resistance of a bipolar transistor; 101, 102, 103 and 104 insulative films to isolate elements, electrodes, and wirings, respectively; and 200 an electrode formed by metal, silicide, polycide, or the like.
The substrate 1 is formed as an n type by doping impurities of phosphorous (Ph), antimony (Sb), arsenic (As), or the like or as a p type by doping impurities of boron (B), aluminum (Al), gallium (Ga), or the like. The buried region 2 is not always necessary. The n.sup.- region 3 is formed by an epitaxial technique or the like. Boron (B), gallium (Ga), aluminum (Al), or the like and germanium (Ge) are doped in the base region 4. As an emitter region 5, polysilicon which was formed by a low pressure chemical vapor deposition (LPCVD) process or the like is used.
In such a conventional HBT, there is a problem such that when the HBT is made fine (is highly integrated), the peripheries of a current (current which is proportional to the emitter area) which flows from an emitter (E) to a collector (C) and an emitter current exert influences, so that a current flowing in the lateral direction (base horizontal direction) increases.
FIGS. 2 and 3 show examples of a conventional BPT and the current flowing in the lateral direction will be briefly explained.
In a schematic cross sectional view of FIG. 2, there is shown an HBT in which a semiconductor region of a wide gap (as compared with a band gap width of a semiconductor region to form a base and a collector) is used for only an emitter. In FIG. 2, reference numeral 201 denotes an n type semiconductor substrate serving as a collector region; 202 a p type semiconductor region serving as a based region; 203 an insulative layer; 204 an n.sup.+ type semiconductor region serving as an emitter region; and 205 an arrow which diagrammatically shows the flow of a current flowing in the BPT (particularly, in the base region) when the BPT shown in FIG. 2 is driven. As shown in FIG. 2, although the current flows in the vertical direction in FIG. 2, the current flow has an extent in the lateral direction in the base region.
A schematic cross sectional view of FIG. 3 shows an example in which a semiconductor of a narrow gap (a gap width narrower than a band gap width of a semiconductor region to form the collector and emitter) is used in the semiconductor region to form the base.
In FIG. 3, reference numeral 301 denotes an n.sup.+ type silicon region; 302 an n type silicon region serving as a collector region; 303 a p.sup.+ type silicon germanium (Si.sub.1-x Ge.sub.x) region serving as a base region; 304 an n type silicon region serving as an emitter region; 305 a p.sup.+ type silicon region to electrically connect the base region with an electrode 306; 307 an n.sup.+ type silicon region to electrically connect the emitter region with an electrode 308; 309 an arrow diagrammatically showing the flow of a current which flows when a double hetero BPT shown in FIG. 3 was driven; and 310 an insulative layer.
As shown in FIG. 3, although the current flows in the vertical direction in FIG. 3, the current flow has an extent in the lateral direction in the base region.
That is, the carriers which were implanted from the emitter region are not effectively shut out and a one-dimensional reduction of a current amplification factor occurs.
That is, a current amplification factor h.sub.FE of the HBT by an inherent current in the vertical direction which flows between the emitter and collector decreases. A similar problem occurs even in the BPT of the homo junction.
When an emitter area is large, a change in dimensions of the emitter area does not substantially exert any influence on the h.sub.FE. However, when the emitter area is made fine, a change in area appears as a change in h.sub.FE of the HBT due to the effect of the peripheral length. In the case of the HBT, when h.sub.FE decreases, an amount of carriers which are implanted into the emitter region suddenly decreases and the recombination current in the base becomes the dominant term of I.sub.B. Therefore, an increase in current in the lateral direction is extremely important.
In a photoelectric converting apparatus, when h.sub.FE decreases, a readout gain from the sensor using the BPT first decreases and a signal voltage drops. That is, this is because the current driving capability of the BPT deteriorates. Then, fixed pattern noises (FPN) increase. Therefore, the signal to noise (S/N) ratio which is most significant in the photoelectric converting apparatus remarkably deteriorates. On the other hand, since the current driving capability of the BPT deteriorates due to a decrease in h.sub.FE, the switching speed and the response speed deteriorate.
Since an influence by the dimensions in the case where the area is made fine appears in h.sub.FE, a variation in h.sub.FE when the sensor element was made fine in correspondence to the high density remarkably increases the photoelectric conversion noises, so that a variation in sensor elements increases and the sensor cannot be used. That is, an aperture ratio of the sensor also decreases, the signal decreases, a variation increases, and the S/N ratio remarkably deteriorates.